b) You have to use a circumvention. Set the dim_left and dim_right attribute of the menu to "0". We will solve this problem ....

>> Menu should be centered at the bottom of the window

The menu is the first item of the window, so it is positioned at x=0 and y=0 automatically. Please use a gap Widget to move the menu to the bottom position. The alignment = "bottom_center" attribute mean, that the position of the menu elements is bottom_centered.

>> Selected item should be always in the middle
You can use the fixed_pos="4" attribute of the menu widget to set the highlighted element.